Was your transplanted hair transplanted into places where existing hair might still fall out?
Have you been taking preventative treatments to try and prevent further loss or slow it down (rogaine, finasteride)?
It's also possible you're experiencing a seasonal shed, which many transplant patients do, in which the hair in the transplanted region looks a little thinner for a season while shedding.
It's also possible for a careless doctor to graft donor hair from outside of the safe zone into your hair and it will be susceptible to miniaturization, more common mistake in FUE than FUT.
And of course in some instances regrettably some people don't have a DHT proof donor area, and it thins with age.
All possibilities to consider, if you're concerned I'd reach out to your doctor or a new one for a consult.